1. Field of Invention
The present invention relates to granular coated particles suitable for use as an agricultural fertilizer, and in particular to granular coated particles suitable for use alone or in combination with other granular particles as a nitrogen-source fertilizer and/or a potassium-source fertilizer. The present invention further relates to a process for preparing the aforementioned granular coated particles, and to a coating and cooling drum arrangement adapted for use in the process.
2. Description of Related Art
Generally, conventional nitrate-containing fertilizers can be classified within one of two categories. The first category relates to fertilizers prepared from nitric acid, which is chemically reacted with, for example, ammonia and/or at least one member selected from the group consisting of phosphate rock, potassium chloride, and limestone. The second category relates to naturally-occurring fertilizers that are either used in their natural state or pretreated prior to use by, for example, purification or metal ion exchange, such as the conversion of sodium nitrate to potassium nitrate.
There are many advantages to the use of nitrates as fertilizers. Nitrogen in nitrate form is more readily available to crops than ammonia-based fertilizers such as urea or ammonium sulfate. Since most crops are more apt to ingest nitrogen in the nitrate form, ammoniacal nitrogen often must be converted to nitrate in the soil before becoming effective as a nitrogen-source fertilizer. For this reason, nitrate fertilizers historically have been the most popular nitrogen fertilizers in most European countries and many other temperate zone countries.
However, there are also profound disadvantages to the use of nitrates as fertilizers. Nitrate fertilizers are diluted in terms of nutrient composition. Of the common nitrate fertilizers, ammonium nitrate has the greatest nitrogen concentrations of about 35% by weight, while sodium nitrate (16.5 wt %), potassium nitrate (13.9 wt %), and calcium nitrate (17.1 wt %) have still lesser nitrogen concentrations. Since nitrate fertilizers have relatively modest nitrogen concentrations, application of large amounts of nitrate fertilizers are often required in practice in order to provide an appropriate amount of nitrogen. This in turn increases the transportation costs associated with the use of nitrate fertilizers.
There are additional reasons why conventional ammonium nitrate fertilizers are not as cost effective as other fertilizers. The cost of materials associated with the production or purchase of conventional ammonium nitrate fertilizers also can have an especially profound financial effect in the agricultural market, where profit margins are often extremely narrow and strongly influenced by such material costs. The production costs associated with the preparation of conventional ammonium nitrate fertilizers are relatively high. Processes for preparing conventional ammonium nitrate fertilizers typically involve multiple steps, including the separate preparation of both the ammonium and metal nitrate constituents, followed by the steps of reacting the constituents to form a melt and granulating or prilling the resulting melt after a concentration step.
In addition to its associated costs, conventional ammonium nitrate fertilizers are moderately explosive. Recently, there has been increasing political pressure directed towards enacting governmental regulations to restrict the sale and use of such potentially dangerous products. Further, these fertilizers are typically devoid of sodium, which is instrumental in preventing soil acidification resulting from nitrification of the soil by ammonium ions.
In order to overcome excessive costs and other problems associated with nitrate fertilizers, it has been proposed to blend nitrate with other granular particles formed of fertilizers having a higher nitrogen concentration than nitrate. For example, urea, which has a nitrate concentration of about 46.3 wt %, has been proposed as a suitable fertilizer for blending with nitrate fertilizers. However, ammonium nitrate granules cannot be blended effectively with urea granules due to the extreme hygroscopicity (critical humidity 18%) of the resultant blend. Accordingly, when ammonium nitrate and urea are mixed, it is usually done to form a liquid fertilizer solution. Blends of separately prepared urea granular particles and metal nitrate granular particles are also plagued by various problems associated with the segregation of one type of particle from the other. High concentrations of one or the other type of the granular particle can result from differences in dimension, shape, and/or specific gravities of the particles. The variable, non-uniform distribution of particles can deleteriously impact the effectiveness of the blended fertilizer in use.
A need therefore exists to provide granular particles adapted for use alone or in combination with other fertilizer particles as a nitrate-containing agricultural fertilizer which affords a uniform distribution of urea and metal nitrate. A need further exists to provide granular particles with an increased nitrogen concentration in comparison to metal nitrate without sacrificing effectiveness. This improves the transportation economics, avoids segregation problems associated with blends, provides a nitrogen ion source for near instant availability of nitrogen to crops, reduces the explosiveness of the fertilizer, and provides a neutralizing sodium source.